1. Field of the Invention
The present invention relates to the conversion of chemical energy to electrical energy, and more particularly, to an alkali metal electrochemical cell having a positive electrode preferably comprising a a transition metal oxide calendered into electrode blanks or plates for subsequent processing into an electrode.
2. Prior Art
Related U.S. Pat. Nos. 5,435,874 and 5,571,640, both to Takeuchi et al., which are assigned to the assignee of the present invention and incorporated herein by reference, are directed to a process for manufacturing a free-standing sheet of cathode material. The process involves first adjusting the particle size of the cathode active material to a useful size followed by mixing with binder and conductivity enhancing additives suspended in a solvent to form a paste. The paste is fed into a series of roll mills which calender the paste into a sheet form. Alternatively, the paste is first pelletized before being subjected to the calendering step. The resulting cathode sheet material is dried and punched into blanks that are subsequently contacted to a current collector to form an electrode.
The step of calendering or compacting the paste or, alternatively, the pelletized cathode active material, is performed by two to four calender mills which serve to sequentially press the active admixture into a tape of a free-standing sheet having a thickness in the range of about 0.004 inches to about 0.020 inches. The sequential calendering step is performed by running the tape from one roll mill to the next with the roll mills contacting the cathode active material along a single direction of travel or direction of contact.
The preferred cathode active materials described in the Takeuchi et al. patents are silver vanadium oxide (SVO) and copper silver vanadium oxide (CSVO), which are coupled with a lithium anode and activated by a nonaqueous electrolyte to provide a cell. These high energy density cells are particularly useful as the power source for an implantable medical device such as a cardiac defibrillator.
The present invention relates to an improved process for providing blanks or plates of electrode active material wherein the tape made by the prior art process is subsequently calendered in a secondary rolling step, substantially orthogonal to the direction at which the first calendering step occurred or at a second direction, opposite the first. This secondary calendering step yields an electrode admixture having a surface area greater than that which is provided by the prior art method having the active material calendered in only one direction, regardless of the number of sequentially reduced gaps the material is run through. The resulting calendered electrode admixture is useful for constructing electrodes for both primary and secondary electrochemical cells.
These and other aspects of the present invention will become more apparent to those skilled in the art by reference to the following description and to the appended drawings.